1. Field of the Invention
The present invention relates to an image forming apparatus and, more specifically, to an image-forming apparatus for forming an image with an enhanced outline.
2. Description of the Prior Art
Generally, outlines of images contain much information and significantly show the characteristics of the images. Therefore, the outlines are important parts in images. In addition, binary processed images of outlines drawn out enable easier handling than normal images having variable density in various processes such as in image-identification. Therefore, the binary processing of the outlines of images is very effective in association with pattern recognition of the image, enhancement, and so on. In addition, a colored outline provided around a black pattern by repeating two copying operations is useful to attract attention, and outlines are useful to provide patterns of different colors produced by painting within respective patterns with different colors.
The applicant of the present invention has proposed methods for forming a positive outline image from a positive original in U.S. Pat. Nos. 4,794,062 and 4,792,775, both issued on Dec. 27, 1988. In these methods, a scorotron charger is provided between an exposing position and a developing position, and provides charges of the polarity opposite to the charges applied on a surface of a recording medium during a charging process. The grid voltage of the scorotron charger is set sufficiently lower than the potential of a latent electrostatic image formed on a surface of the recording medium and lower than the developing bias voltage applied to the developing device.
When such a capability is added to a copying machine, operation keys and display devices are accordingly added, causing manufacturing costs to soar. In addition, the operation panel becomes complicated, rendering the operation troublesome for an operator.